JP2 Signature format

User's guide:

JPEG 2000 Signature Format: Documentation & Recovery Example

JPEG 2000 (extensions: *JP2, *.JPA, *.JPM, *.JPX) is an image compression standard and coding system.
It was created by the Joint Photographic Experts Group committee in year 2000 with the intention of superseding their original discrete cosine transform-based JPEG standard with a newly designed, wavelet-based method.
The standardized filename extension is .jp2 for ISO/IEC 15444-1 conforming files and .jpx for the extended part-2 specifications, published as ISO/IEC 15444-2.

Second chunk must be of type "ftyp" and has a sub-type at offset 8. JPEG 2000 defined by sub-type which must be one of values: "jp2 "(file type *.JP2), "jp20" (file type *.JPA), "jpm " (file type *.JPM), "jpx " (file type *.JPX).